This invention relates to a pilot signal cancelling circuit for an MPX demodulator for cancelling pilot signal components contained in stereophonic signals obtained by dividing a composite signal.
In demodulating an FM signal in the pilot tone system, a pilot signal of 19 KHz is selectively detected from a composite FM signal, a sub-carrier signal of 38 KHz is obtained from the pilot signal, and right (R) and left (L) stereophonic signals are obtained from the composite signal by utilizing the sub-carrier signal as a switching signal. The composite signal also contains the pilot signal, however, whereby the latter becomes mixed into the right and left stereophonic signals through the switching operation of the sub-carrier signal.
As is indicated in FIG. 1, the pilot signal (a) is gated by the sub-carrier switching signals (b) and (c) into signals having waveforms (d) and (e), which are mixed into the right and left stereophonic signals. As the signal components (d) and (e) would otherwise lower the S/N ratio of the normally demodulated right and left stereophonic signals, they are typically eliminated by using a trap or a low pass filter. This method is disadvantageous, however, in that it may lower the fidelity characteristics of the normally demodulated stereophonic signals.
In order to overcome this difficulty, a pilot signal cancelling circuit generally indicated by reference numeral 9.sub.2 in FIG. 2 has been proposed, as described in Japanese Patent Application No. 106393/1975. Signals of 38 KHz and 19 KHz whose phases are in synchronization with the phase of the 19 KHz pilot signal (FIG. 1(a)) in the composite signal are obtained by a phase lock loop (PLL) 2. With the aid of the 38 KHz signals, the composite signal is synchronously detected by a multiplexer MPXR 3 wherein the composite signal is divided into right and left stereophonic signals each containing the pilot signal. At the same time, the zero phase angle pilot signal (19 KHz .angle. 0.degree. ) from the PLL 2 is inverted by a phase inverting circuit 6, and the inverted signal (FIG. 1 (f) ) is divided into right and left signal components (FIG. 1 (g) and (h) ) by switching circuits 7 and 8 in response to the 38 KHz signals. These signal components are then added to the right and left stereophonic signals from the MPXR 3 by adders 4 and 5, wherein they cancel the pilot signal components (d) and (e).
Such a pilot signal cancelling circuit is not completely effective, however, in cases where the level of the pilot signal contained in the composite signal varies or fluctuates, since the level of the pilot signal cancelling components obtained from the PLL 2 is fixed in magnitude. Thus, if the level of the pilot signal contained in the composite signal varies in response to radio wave propagation conditions or between different broadcasting stations selected, the cancelling operation effected by the adders is sometimes incomplete.